Angela Merkel has said it is in Germany’s national interest for the UK to remain in the European Union, as David Cameron embarked on a round of last-minute diplomacy ahead of a crucial summit on Thursday.

Shortly before a summit in Brussels where David Cameron’s reform proposals will be thrashed out, the German chancellor appealed for more sympathy and understanding for the British position.

In London, Cameron undertook a round of telephone calls with EU leaders, and held talks in Downing Street with the London mayor, Boris Johnson, one of the most prominent figures in Cameron’s party yet to make his position known on the EU deal.

After the meeting, a source close to the mayor said he would not reveal his views until after Cameron returned from the summit: “The mayor will make everything abundantly clear by the end of the week if the prime minister gets a deal on Friday. He is genuinely undecided.”

In Berlin, Merkel told German delegates that the British prime minister’s demands were “comprehensible and justified”. Keeping Britain in the EU, the chancellor told the German parliament, was “not just in Britain’s but also in Germany’s interest, and that of Europe as a whole”.

She said she shared Cameron’s view that non-eurozone countries should not be sidelined and that member states should be able to protect their benefits systems. “There is no point of dissent between the UK and Germany as far as social systems are concerned,” Merkel said.

However, disagreements with other countries remain. Speaking after a meeting of his government on Wednesday, Bohuslav Sobotka, the prime minister of Czech Republic, said he was willing to work towards an EU deal for Britain but not at the expense of his country’s citizens.

Merkel: Cameron's EU demands are 'justified and necessary' – live

Read more

Sobotka said the limit on benefits for workers that Britain is requesting should not be applicable to those who are already working in Britain, only for workers arriving in the future.

On Tuesday, the scale of the challenge facing Cameron became clear when Poland, Slovakia, Hungary, and the Czech Republic collectively rejected current proposals on curbing child benefits for their migrant workers in western Europe, a key demand in his campaign.

Merkel said it was vital that Britain’s reformed position in the EU did not mean that it could stop the bloc from acting independently.

“Our experience of the European household debt crisis has shown us how quickly steps towards further integration can be required, and I remind you that no long-term solutions have been found for the problems that become visible during the crisis,” she said.

The principles of freedom of movement and non-discrimination were “not open for discussion”, the chancellor said. What was important was that further integration remained “a possibility, but not an obligation”.

Merkel added that all the German government could do was provide the British government with the best possible arguments for staying in the bloc. “In the end, the British voters will decide,” she said.

Donald Tusk, the European council president, was due to publish a final draft of his proposals for a new settlement for the UK ahead of the Thursday summit. Tusk said on Tuesday that EU leaders would have to go an “extra mile” to reach an agreement.